Recently, in a computer terminal device, a plain paper copying apparatus, a picture record printer and the like, optical printers utilizing a combination of fine light emitting points and photosensitive members have been developed with attention directed to advantages thereof such as high-speed, high resolution, fineness in printing and quietness of the devices. Such devices are called as a laser printer, a light emitting diode printer (LED printer) and the like depending on types of light sources employed therein.
LED printers according to the prior art of interest are disclosed in U.S. Pat. Nos. 3,850,517 (Stephany et al.), 4,318,597 (Kotani et al.) and Japanese patent laying-open gazette No. 55770/1984 (Yoshida) etc. These are now briefly described with attention to optical printing heads and driving methods.
First, the LED printer disclosed in U.S. Pat. No. 3,850,517 has a decoder in each block of light emitting diode arrays and employs codes as printing data. Therefore, although the transfer speed of the printing data itself is high, a number of decoders and printing timings after decoding of the printing data are required, and further, this printer cannot be applied to printing of those not included in characters, e.g., picture images.
Next, the LED printer disclosed in U.S. Pat. No. 4,318,597 comprises light emitting diode arrays alternately arranged in the so-called zig-zag manner and printing data are formed by serially transferred dot information, and hence it is suitable for printing of picture images and the like. However, the data are basically transmitted to shift registers in a series manner, and one of the arrays is delayed by a memory in compliance with the zig-zag arrangement. Consequently, this LED printer requires a long data transfer time, and further requires two systems of printing timings synchronized with the cross scanning direction.
Lastly, the LED printer disclosed in Japanese patent laying-open gazette No. 55770/1984 is of a typical dynamic lighting type, and hence common electrodes are separated for each of light emitting diode arrays and a number of timing signals for light emitting points are required with respect to a horizontal scanning line.
A light emitting diode array is provided with light emitting areas (dots) being in alignment, which correspond in size and position to dots to be printed in the 1:1 ratio. On the other hand, one light emitting diode cannot form all dots over the entire length of the main scanning direction, and hence a plurality of short light emitting diode arrays are employed in alignment. Therefore, dot pitches are required to be constant in joints of the light emitting diode arrays. However, when the respective ones of the light emitting diode arrays are placed on separate substrates to be arranged in two lines as in the aforementioned materials, alignment of the optical images (i.e., supporting of the light emitting diode arrays and adjustment of the optical systems) is extremely difficult. Further, connection of common line independently provided for each of the light emitting diodes for dynamic driving complicates operation for the alignment is complicated. Placing a plurality of light emitting diode arrays on one substrate without regard to the connection of common line would be a method of solving the problem of difficulty in such work. More preferably, the plurality of light emitting diode arrays are arranged in one line.
Further, an LED printer has such an advantage that a head thereof is a solid element which enables high-speed printing, wherefore high-speed data processing is required. However, if extreme high-speed is required for the data transfer speed and the transfer time is lengthened, the said advantage cannot be efficiently applied while integrated circuit elements to be used are restricted, and the power consumption is increased in case of, e.g., a high speed TTL, leading to inconvenience.
In addition, intensity of the head shows dispersion depending on the synergistic effect of the light emitting characteristics of the light emitting diode arrays and the output characteristics of driving elements, whereas no definite countermeasures therefor are indicated in the aforementioned prior art examples. The present invention has been proposed in consideration of these points.